1. Field of the Invention
The present invention relates to methods and apparatus for drilling wells and, more particularly, to a reamer and corresponding method for enlarging the drift diameter and improving the well path of a well bore.
2. Description of the Related Art
Extended reach wells are drilled with a bit driven by a down hole motor that can be steered up, down, left, and right. Steering is facilitated by a bend placed in the motor housing above the drill bit. Holding the drill string in the same rotational position, such as by locking the drill string against rotation, causes the bend to consistently face the same direction. This is called “sliding”. Sliding causes the drill bit to bore along a curved path, in the direction of the bend, with the drill string following that path as well.
Repeated correcting of the direction of the drill bit during sliding causes friction between the well bore and the drill string greater than when the drill string is rotated. Such corrections form curves in the well path known as “doglegs”. Referring to FIG. 1a, the drill string 10 presses against the inside of each dogleg turn 12, causing added friction. These conditions can limit the distance the well bore 14 can be extended within the production zone, and can also cause problems getting the production string through the well bore.
Similar difficulties can also occur during conventional drilling, with a conventional drill bit that is rotated by rotating the drill string from the surface. Instability of the drill bit can cause a spiral or other tortuous path to be cut by the drill bit. This causes the drill string to press against the inner surface of resulting curves in the well bore and can interfere with extending the well bore within the production zone and getting the production string through the well bore.
When a dogleg, spiral path or tortuous path is cut by a drill bit, the relatively unobstructed passageway following the center of the well bore has a substantially smaller diameter than the well bore itself. This relatively unobstructed passageway is sometimes referred to as the “drift” and the nominal diameter of the passageway is sometimes referred to as the “drift diameter”. The “drift” of a passageway is generally formed by well bore surfaces forming the inside radii of curves along the path of the well bore. Passage of pipe or tools through the relatively unobstructed drift of the well bore is sometimes referred to as “drift” or “drifting”.
In general, to address these difficulties the drift diameter has been enlarged with conventional reaming techniques by enlarging the diameter 16 of the entire well bore. See FIG. 1a. Such reaming has been completed as an additional step, after drilling is completed. Doing so has been necessary to avoid unacceptable increases in torque and drag during drilling. Such additional reaming runs add considerable expense and time to completion of the well. Moreover, conventional reaming techniques frequently do not straighten the well path, but instead simply enlarge the diameter of the well bore.
Accordingly, a need exists for a reamer that reduces the torque required and drag associated with reaming the well bore.
A need also exists for a reamer capable of enlarging the diameter of the well bore drift passageway and improving the well path, without needing to enlarge the diameter of the entire well bore.